1. Field of the Invention
The present invention relates generally to cleaning of a semiconductor wafer and, more particularly, to physical confinement of a liquid meniscus applied on the surface of the semiconductor wafer during a cleaning process.
2. Description of the Related Art
It is well-known in the art that there is a need to clean and dry a solid surface, such as a semiconductor wafer, that has undergone a fabrication operation which leaves unwanted residues on the solid surface. Examples of such fabrication operations include plasma etching (e.g., via etch or trench etch for copper dual damascene applications) and chemical mechanical polishing (CMP). Various cleaning processes involve applying a liquid chemistry to the semiconductor wafer as a meniscus and removing the liquid chemistry along with the released contaminants. It is important to maintain the meniscus over the surface of the semiconductor wafer so that the liquid chemistry can work to release the particle contaminants from the surface of the semiconductor wafer. Conventional cleaning processes use proximity heads to apply the liquid chemistry to one side or to both sides of the wafer and confining the meniscus through large air flow. Vacuum is then used to provide sucking action to entrain the liquid chemistry. The conventional confinement of liquid chemistry using large air flow has its own disadvantages. For instance, the liquid chemistry is exposed to the large air flow resulting in substantial loss of the liquid chemistry due to evaporation. Some of the liquid chemistry used in the cleaning process is very expensive and any loss of liquid chemistry adds to the cost of cleaning.
Evaporation of the liquid chemistry in conventional cleaning processes is a serious issue, especially when using proximity heads. Specifically, due to high ambient air flow through the proximity heads, it is difficult to control evaporation loss of the liquid chemistry. Additionally, in order to improve the cleaning process, it is common to introduce the liquid chemistry at a higher temperature, typically about 30 degrees Celsius to about 60 degrees Celsius. Liquid chemistry loss can dramatically increase when liquid chemistry is applied at a higher temperature. This is due to the fact that vapor pressure exponentially increases with temperature and as evaporation is directly related to vapor pressure, evaporation also increases. As a result, the amount of liquid chemistry that can be reclaimed for reuse dramatically decreases.
Another factor for consideration is the effect the high temperature liquid chemistries have on the conventional cleaning apparatus, such as chemical heads, used in supplying these liquid chemistries. Most of the conventional cleaning apparatus operate optimally at room temperature. However, static temperature gradient that naturally develops because of higher temperature of the liquid chemistries cause these cleaning apparatus to deform resulting in mediocre operation of the apparatus during cleaning.
Another disadvantage of the use of air for confining the meniscus is the cost of generating vacuum in the presence of this large flow of air. The design requirement for generating the vacuum has to take into consideration this large air flow requirement so as to provide an effective tool for cleaning.
Moreover, evaporation can result in significant changes in the cleaning process by liquid chemistry due to the chemical depletion or change in concentration of the liquid chemistry. Chemical depletion occurs when the ambient air flow mixes with the hot liquid chemistry resulting in vapor that is saturated with air and components of the liquid chemistry making it hard to isolate and reclaim the liquid chemistry. Excessive concentration of chemicals, on the other hand, commonly results with the use of proprietary chemistry. Proprietary chemistry contains non-volatile components and, if the proprietary chemistry is aqueous-based, evaporation causes the concentration of non-volatile components to increase over time. This increase in the concentration of non-volatile components can adversely affect the cleaning performance of the liquid chemistry. Moreover, if the concentration of the liquid chemistry increases too much, there might be significant damage to the semiconductor wafer.
In view of the foregoing, there is a need for an alternate solution that avoids use of air flow to confine the liquid meniscus. It is in this context that embodiments of the invention arise.